For the polymerization of olefins, to which the method and equipment according to the invention relates, the Ziegler-Natta catalyst system is commonly used, which consists of a so-called procatalyst and a cocatalyst. The procatalyst is based on a compound of a transition metal belonging to any of the groups IVA-VIII (Hubbard) of the periodical table of the elements and the cocatalyst is based on an organometallic compound of a metal belonging to any of the groups IA-IIIA (Hubbard) of the periodical table of the elements. The procatalyst can also comprise a carrier, on which the transition metal compound is layered, and an internal electron donor improving and modifying the catalytic properties thereof.
The reacting of the transition metal compound onto the surface of the carrier is carried out between a solid phase carrier being and most frequently a liquid transition metal compound or a transition metal compound in a liquid. Thus this stage of the procatalyst preparation includes the operations where the starting materials are fed into the reaction vessel and suitable reaction conditions are created therein to separate the unreacted liquid from the product and to wash and dry the product.
The carrier layered with the transition metal compound is available for use as a dried powder for the polymerization of olefins, whereby the combination with the co-catalyst is carried out before or at the start of the polymerization operation. The above-mentioned operations for the preparation of a procatalyst are usually carried out in different units and thus the intermediate products must be transferred from one unit to another during the production. As both the reaction components and the reactions are very sensitive to impurities, such as the oxygen and moisture of the air, such transfer operations can result in low and unequal quality. Moreover, the various reaction and washing operations succeeding each other require that the created solid precipitate sinks to the bottom of the vessel, whereby the agitation must be stopped and the liquid siphonated away from the surface of the precipitate. Such an operation leaves an abundance of activation and washing liquid in the precipitate, for which reason its separation and washing effect is weak and many washing stages are required before a required purity grade is achieved.
Accordingly, an unreasonable amount of time and washing chemicals are spent in the present operations and, moreover, a part of the usable solid material disappears in the siphonation stage as it is dispersed in the liquid to be removed.
The drying stage can be very problematic since the polymerization catalyst, being sensitive to the oxygen and moisture in the air, can, in the absence of a protecting liquid layer, be destroyed, unless the operations are carried out in a very inert, dry and oxygen-free atmosphere. When mechanical driers are used the danger exists that the fragile catalyst particles are broken and imperceptible air leakages easily occur during the vacuum drying which destroy the catalyst.
To solve the above-mentioned problems it is known to use the method according to the Finnish patent No. 83329 and a multi-function reactor, in which all or an essential part of the transfer, reaction, washing and drying operations are carried out in a multi-function reactor furnished with a mixer and a bottom sieve net which is permeable to as well the liquid and finely-divided solid materials separated from the catalyst as to the inert protective and/or drying gas conducted to the reactor. Thus, the preparation stages of the supported polymerization catalyst can be carried out in the same reactor, when the reactor is a container furnished with a mixer having at its bottom a sieve net permeable to the materials to be separated from the solid procatalyst product. The obtained procatalyst particles, on the other hand, remain on the sieve net from where they can be recovered for use.
This prior art method and device still has, however, many disadvantages. While, in the decantation operation, the middle size and thus useful procatalyst particles might be dispersed in the solution to be decanted and thus be lost, in the filtering operation, the harmful, finely-divided material may remain in the raw-procatalyst precipitate, resulting in a procatalyst which contains much of the harmful, finely-divided material. Thus, the problem is that the non finely-divided material is first sedimentated onto the sieve net and then acts as a finer sieve than the sieve net itself when the reaction liquid is pressed or sucked through the sieve net. Finely-divided material remains in the procatalyst precipitate and only clear solution comes through the sieve.
Another disadvantage in the above-mentioned conventional method and device is that new, harmful, finely-divided procatalyst is created during the mixing in connection with the washing. As it is hard to remove, among other, the above-mentioned finely-divided material by filtering, the washing generally has comprised several washing cycles, each implying mechanical strain on the fragile procatalyst e.g. in the form of the mixing. The more washing cycles including mixing, the more the procatalyst is broken and the more harmful, finely-divided material is created.
New finely-divided material is also created in connection with the drying where mixing is also needed to prevent the formation of a tight product cake which prolongs the drying. During the drying the procatalyst breaks particularly easily because there in no liquid left protecting its particles. Thus, for example, a mechanical mixing carried out by a propeller very easily breaks the catalyst.
If in the above-mentioned conventional method and device the mixing during the drying is left out, the alternative to feed inert gas, such as nitrogen, through the procatalyst cake which has settled on the sieve. As was mentioned before, the drying time is thereby very long, of the order of 1 to 2 days (24 h), and the amount of the drying gas used is very large. Nitrogen, which is generally used as the drying gas, always contains a little oxygen, hydrogen, carbon dioxide, etc., which react with the procatalyst and decrease its activity. It can be said that according to the conventional technique the drying can be carried out either so that harmful, finely-divided procatalyst is created or so that a part of the procatalyst is poisoned by the impurities of the drying gas.
Moreover, the capacity of the conventional reaction vessel is very small, because usually a large excess of transition metal compound is needed to keep the content of the reaction products of the equilibrium reactions between the procatalyst particles and the transition metal compound so low that enough product is being produced.
The aforesaid shows that the purpose of the present invention is to remove the finely-divided material from the product, to prevent the formation of new finely-divided material both in connection with the washing and the drying, to prevent the procatalyst from being poisoned when drying with inert gas and to increase the capacity of the reaction vessel when reacting carrier particles and a transition metal compound, All these aims have to be carried out so that the other properties of the procatalyst obtained will remain at least as good as before. The procatalyst particles must be useful with respect to their form, their chemical structure, their activity and their stereospecificity and they must produce useful olefin.